This invention relates to a gas-cooled dynamoelectric machine and in particular to an integral composite nozzle-shield assembly used in directing the flow of cooling gas within a large turbine-driven generator.
Dynamoelectric machines such as large turbine-driven generators utilize a gas such as hydrogen for cooling the stator core and portions of a rotor mounted within a central bore of the stator core. The cooling gas is circulated by a fan attached to the rotor and by pumping action of the rotor itself and is directed through passages in the rotor and stator core and recirculated through gas coolers usually located in the upper portion of the machine. The general direction of coolant flow may be in the "reverse" sense, as disclosed in U.S. Pat. No. 3,739,208 to Shartrand and U.S. Pat. No. 4,039,872 to Armor et al, both assigned to the assignee of the present invention, wherein gas is drawn into the fan from the end of the gas gap between rotor and stator core, pumped upward to a gas cooler, then split into a first stream which is directed through generally radial stator core inlet cooling passages and into the gas gap for pickup and discharge at the rotor surface and a second stream which is directed from the cooler to passages in the rotor end portion. The gas is then returned to the fan from the rotor end portion cooling passages and from generally radial stator core outlet cooling passages. Alternatively, the gas coolant may circulate in the "forward" direction from the fan through stator core inlet cooling passages to the gas gap for pickup at the rotor surface and also from the fan to passages in the rotor end portions, and then back to the gas gap and radially outward through stator core outlet cooling passages and to the gas cooler.
In either case, in order to provide a smooth passage for flow between the fan and gas cooler and to permit the fan to develop pressures adequate to pump gas through the cooler and coolant passages, a nozzle ring is mounted over the fan blade tips in sealing relationship with the fan blades. The nozzle ring is typically bolted to an annular shield which extends radially outward from the nozzle ring and in turn is bolted to a frame member of the stator, and this combination, together with the end portion of the generator casing, channels the gas coolant between the cooler and fan. In many dynamoelectric machines both nozzle ring and shield are formed of metallic components such as steel; however, in the above-mentioned U.S. Pat. No. 4,039,872 to Armor et al the deleterious effect on performance of a generator of eddy currents induced in magnetically susceptible metallic nozzle guide vanes and their mounting assembly by stray flux from the rotor and the end turns of the stator winding bars was recognized, and accordingly the guide vanes and nozzle rings described in U.S. Pat. No. 4,039,872 were formed of electrically non-conductive non-magnetic material such as fiberglass. While this limited use of fiberglass is effective to eliminate eddy-current heating in the nozzle ring and guide vanes of a generator and thus provides a significant performance benefit, the retention of a separate metallic shield (e.g., member 43 in FIG. 1 of U.S. Pat. No. 4,039,872) to which the nozzle ring is attached entails certain drawbacks. For example, since the shield is located near the source of stray flux, it too, if metallic, is susceptible to eddy-current heating which penalizes efficiency of the dynamoelectric machine. Moreover, the proximity of the shield to the end turns of the stator winding bars presents some risk of electrical flashover or arcing which could under certain circumstances occur between a metallic shield and the stator end turns if bar insulation failed; the likelihood of a flashover, though remote, rises somewhat as machine power ratings tend to increase and hence result in higher stator winding currents and voltages during operation. Finally, the use of a separate metallic shield results in a heavy member whose manufacture and field assembly to a non-metallic nozzle ring is complex and costly.
Accordingly, it is an object of the present invention to provide an integral nozzle-shield assembly for a dynamoelectric machine which is not subject to eddy-current heating or electrical flashover.
It is another object of the invention to provide a nozzle-shield assembly for a dynamoelectric machine which includes at least two arcuate sections and which has substantially uniform stiffness circumferentially.
A further object of the invention is to provide a nozzle-shield assembly for a dynamoelectric machine which is lightweight and easy to fabricate and install.